The immediate aim of the project is to determine the exact molecular structures of a series of biologically important steroids and steroid complexes, particularly those which are significant in the study and treatment of cancer, employing X-ray crystallographic techniques. By "exact" molecular structure is meant: (a) the bond lengths and bond angles, (b) the conformations of the individual rings, (c) the overall conformation of the steroid nucleus, and (d) the relative orientations of possible active substituents. An Atlas of Steroid Structure, which presents exact structural details for approximately 200 steroidal compounds is in press. This Atlas will facilitate comparative studies of steroid molecular structures and the understanding of steroid structural-functional relationships. The potentials for gathering structural information are being expanded through the development of direct methods of structure determination. Comparative analysis of the existing structural data is in progress in order to elucidate (a) the relationships between molecular conformations in solid and in solution, (b) relative potential energies of observed conformations, (c) interactions of steroids with their immediate environment, (d) mechanisms of certain enzymic reactions involving steroid substrates, (e) relationships between steric features and steroid-protein binding characteristics, and (f) correlations between conformational features and pharmacological activity. The long- term goal of this project is to establish structural reasons, such as the occurrence of extremely long or short bonds, ring distortions, stereospecific conformations, functional side chain orientations, or particular molecular stackings for the general biological specificity of steroids and for the explicit steroidal-endocrine mechanisms which control neoplastic processes. Bibliographic references: Weeks, C.M., Duax, W.L. and Osawa, Y., 17-beta-Chloroacetoxy-2-beta-acetoxy-4-androsten-3-one, C23H31ClO5, Cryst. Struct. Comm., 4, 97-100 (1975). Cody, V., Duax, W.L., Weeks, C.M. and Wolff, M.E., 2,4-Dioxa-5 alpha-androstan-17 beta-o1 Acetate, Acta Cryst., B31, 292-295 (1975).